After legalizing recreational marijuana last year, Colorado voters Tuesday are considering a tax on the drug.

The ballot measure represents the "politics of compromise," said Rep. Jonathan Singer, the Democratic state legislator behind the tax proposal. The tax on marijuana must be high enough to generate revenue for enforcement but low enough to deter consumers from buying the drug on the black market, Singer said.

Voters are considering a 15% excise tax on marijuana producers to pay for school construction, and a 10% special sales tax on consumers to fund marijuana enforcement. In Colorado, any tax increase must get the voters' approval. The taxes could generate an estimated $67 million annually after the first year.

If the measure passes, the state legislature has the option to raise the 10% special sales tax to as much as 15% in future years, according to the proposal. The excise tax is capped at 15%.

The legislature could also lower the excise and sale taxes if it appears the taxes are too high and marijuana users are still buying the drug illegally. The magic number for taxing the drug is still anyone's guess.

"The number one truth is, we don't know because no one's done this before," Singer said.

Colorado and Washington are the only states to legalize recreational pot. In Washington, a 25% marijuana tax is levied three times, for producers, processors and consumers. After pass-down costs, the effective tax rate for consumers is 44% in Washington.

The state began accepting applications for retail marijuana locations Oct. 1 and the earliest these locations will open is Jan. 1, 2014.

For a bare-bones enforcement budget, the state needs $5 million to $7 million for its Marijuana Enforcement Division in the state's Department of Revenue. This division is charged with establishing a "seed-to-sale" tracking program to ensure marijuana does not fall in the hands of criminals, children or leave the state, Singer said.

Will black market persist?

Colorado's proposed tax on recreational marijuana is higher than other "sin" taxes in Colorado — such as the tax on cigarettes and alcohol — but the rate is still reasonable, said Allen St. Pierre, executive director of NORML, an advocacy group that supports marijuana legalization.

"Marijuana is a problematic adult commerce, and with problematic adult commerce, one would expect a higher level of taxation and control," St. Pierre said.

The national NORML organization has taken a neutral stance on Colorado's tax proposal. However, the local NORML chapter has opposed the tax rate, agreeing with the 15% excise tax but arguing that the special sales tax is too high.

"This type of sales tax is going to incentivize people to remain on the black market," said Rachel Gillette, executive director of Colorado NORML and an attorney representing marijuana businesses in Colorado.

On top of the state tax, Colorado towns and cities can vote to add an additional local sales tax on recreational pot. So far, a dozen municipalities are considering an additional tax on marijuana, according to the Colorado Municipal League.

Singer said legislators "did their homework" and found that even with the additional taxes, marijuana sold in retail locations will be "well under what you would be paying on the black market," Singer said.

Currently, medical marijuana in Colorado costs about $200 per ounce and is taxed the 2.9% retail sales tax. On the black market in other states, marijuana is selling for double that, Singer said.

Last year, 53% of Colorado voters supported legalizing pot. A Public Policy Polling survey earlier this year predicted an even higher rate of support for the tax proposal — 77% polled supported the 15% excise tax and the 10% special sales tax. Marijuana industry groups have also come out in favor of the proposed tax rate.

Democratic state legislator Dan Pabon for Colorado chaired the committee in charge of implementing the pot legalization. When it came to taxes, Pabon said the committee looked to past taxations on black market products. One example is the move away from illegal digital music downloads on websites like Napster to paid websites like iTunes, he said.

"Consumers want to not be engaging in illegal behavior and are willing to pay a little bit more, and we see that in regulating legal behavior," Pabon said.